Could 'Oumuamua be an extraterrestrial solar sail?

Artist’s impression of the first interstellar asteroid/comet, “Oumuamua”. This unique object was discovered on 19 October 2017 by the Pan-STARRS 1 telescope in Hawaii. Credit: ESO/M. Kornmesser

On October 19th, 2017, the Panoramic Survey Telescope and Rapid Response System-1 (Pan-STARRS-1) in Hawaii announced the first-ever detection of an interstellar asteroid, named 1I/2017 U1 (aka, "Oumuamua). In the months that followed, multiple follow-up observations were conducted that allowed astronomers to get a better idea of its size and shape, while also revealing that it had the characteristics of both a comet and an asteroid.

Interestingly enough, there has also been some speculation that based on its shape, 'Oumuamua might actually be an interstellar spacecraft (Breakthrough Listen even monitored it for signs of radio signals!). A new study by a pair of astronomers from the Harvard Smithsonian Center for Astrophysics (CfA) has taken it a step further, suggesting that 'Oumuamua may actually be a light sail of extra-terrestrial origin.

The study – "Could Solar Radiation Pressure Explain "Oumuamua's Peculiar Acceleration?," which recently appeared online – was conducted by Shmuel Bialy and Prof. Abraham Loeb. Whereas Bialy is a postdoctoral researcher at the CfA's Institute for Theory and Computation (ITC), Prof. Loeb is the director of the ITC, the Frank B. Baird Jr. Professor of Science at Harvard University, and the head chair of the Breakthrough Starshot Advisory Committee.

To recap, 'Oumuamua was first spotted by the Pan-STARRS-1 survey 40 days after it made its closest pass to the sun (on September 9th, 2017). At this point, it was about 0.25 AU from the sun (one-quarter the distance between Earth and the sun), and already on its way out of the solar system. In that time, astronomers noted that it appeared to have a high density (indicative of a rocky and metallic composition) and that it was spinning rapidly.

While it did not show any signs of outgassing as it passed close to our sun (which would have indicated that it was a comet), a research team was able to obtain spectra that indicated that 'Oumuamua was more icy than previously thought. Then, as it began to leave the solar system, the Hubble Space Telescope snapped some final images of 'Oumuamua that revealed some unexpected behavior.

After examining the images, another international research team discovered that 'Oumuamua had increased in velocity, rather than slowing down as expected. The most likely explanation, they claimed, was that 'Oumuamua was venting material from its surface due to solar heating (aka. outgassing). The release of this material, which is consistent with how a comet behaves, would give 'Oumuamua the steady push it needed to achieve this boost in velocity.

To this, Bialy and Loeb offer a counter-explanation. If 'Oumuamua were in fact a comet, why then did it not experience outgassing when it was closest to our sun? In addition, they cite other research that showed that if outgassing were responsible for the acceleration, it would have also caused a rapid evolution in 'Oumuamua's spin (which was not observed).

Basically, Bialy and Loeb consider the possibility that 'Oumuamua could in fact be a light sail, a form of spacecraft that relies on radiation pressure to generate propulsion – similar to what Breakthrough Starshot is working on. Similar to what is planned for Starshot, this light sail may been sent from another civilization to study our solar system and look for signs of life. As Prof. Loeb explained to Universe Today via email:

"We explain the excess acceleration of `Oumuamua away from the sun as the result of the force that the sunlight exerts on its surface. For this force to explain measured excess acceleration, the object needs to be extremely thin, of order a fraction of a millimeter in thickness but tens of meters in size. This makes the object lightweight for its surface area and allows it to act as a light-sail. Its origin could be either natural (in the interstellar medium or proto-planetary disks) or artificial (as a probe sent for a reconnaissance mission into the inner region of the solar system)."

Based on this, Bialy and Loeb went about calculating the likely shape, thickness, and mass-to-area ratio that such an artificial object would have. They also attempted to determine whether this object would be able to survive in interstellar space, and whether or not it would be able to withstand the tensile stresses caused by rotation and tidal forces.

What they found was that a sail that was only a fraction of a millimeter thick (0.3-0.9 mm) would be sufficient for a sheet of solid material to survive the journey through the entire galaxy – though this depends greatly on 'Oumuamua's mass density (which is not well-contrained). Thick or thin, this sail would be able to withstand collisions with dust-grains and gas that permeate the interstellar medium, as well as centrifugal and tidal forces.

As for what an extra-terrestrial light sail would be doing in our solar system, Bialy and Loeb offer some possible explanations for that. First, they suggest that the probe may actually be a defunct sail floating under the influence of gravity and stellar radiation, similar to debris from ship wrecks floating in the ocean. This would help explain why Breakthrough Listen found no evidence of radio transmissions.

Loeb further illustrated this idea in a recent article he penned for Scientific American, where he suggested that 'Oumuamua could be the first known case of an artificial relic which floated into our solar system from interstellar space. What's more, he notes that lightsails with similar dimensions have been designed and constructed by humans, including the Japanese-designed IKAROS project and the Starshot Initiative with which he is involved.

"This opportunity establishes a potential foundation for a new frontier of space archaeology, namely the study of relics from past civilizations in space," Loeb wrote. "Finding evidence for space junk of artificial origin would provide an affirmative answer to the age-old question "Are we alone?". This would have a dramatic impact on our culture and add a new cosmic perspective to the significance of human activity."

On the other hand, as Loeb told Universe Today, 'Oumuamua could be an active piece of alien technology that came to explore our solar system, the same way we hope to explore Alpha Centauri using Starshot and similar technologies:

IKAROS spaceprobe with solar sail in flight (artist’s depiction) showing a typical square sail configuration. Credit: Wikimedia Commons/Andrzej Mirecki
"The alternative is to imagine that `Oumuamua was on a reconnaissance mission. The reason I contemplate the reconnaissance possibility is that the assumption that `Oumumua followed a random orbit requires the production of ~10^{15} such objects per star in our galaxy. This abundance is up to a hundred million times more than expected from the solar system, based on a calculation that we did back in 2009. A surprisingly high overabundance, unless `Oumuamua is a targeted probe on a reconnaissance mission and not a member of a random population of objects."

According to Loeb, that there's also the fact that 'Oumuamua's orbit brought it to within 0.25 AU of the sun, which is a good orbit for intercepting Earth without experiencing too much solar irradiation. In addition, it came to within 0.15 AU of Earth, which could have been the result of orbital corrections designed to facilitate a flyby.

Alternately, he states that it is possible that hundreds of such probes could be sent so that one of them got close enough to Earth to study it. The fact that the Pan STARRS-1 survey barely detected 'Oumuamua at its closest approach could be seen as an indication that there are many other such objects that were not detected, bolstering the case for 'Oumuamua being one of many such probes.

Considering that astronomers recently concluded that our solar system has likely captured thousands of interstellar objects like 'Oumuamua, this opens the possibility for future detections which could help prove (or disprove) the case for an interstellar light sail.

Naturally, Bialy and Loeb acknowledge that are still too many unknowns to say with any certainty what 'Oumuamua really is. And even if it does happen to be a piece of natural rock, all other asteroids and comets that have previously been detected have had mass-to-area ratios orders of magnitude larger than the current estimates for 'Oumuamua.

'Oumuamua as it appeared using the William Herschel Telescope on the night of October 29th, 2017. Credit: Queen’s University Belfast/William Herschel Telescope

That, and the fact that radiation pressure appears to be capable of accelerating it, would mean that 'Oumuamua represents a new class of thin interstellar material that has never before been seen. If true, that opens up a whole new set of mysteries, such as how such material was produced and by what (or whom).

While it has been beyond the reach of our telescopes for almost a year now, 'Oumuamua is sure to remain the subject of intense study for many years to come. And you can bet astronomers will be on the lookout for more of them! After all, "the Ramans do everything in threes," right?

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44 comments

Wouldn't the sail for an object of this size need to be in the hundreds of square Km range? How did we miss that? Also wouldn't its tumble imply that the pilot was completely wasted as they entered the system?

Assuming the possibility of an alien origin means dumping any preconceptions based on our current and/or plausibly imaginable technology.....or at least being a little more realistic with such suppositions~For instance, Oumuamua's sustained tumble, which oddly defies natural damping, may imply that plastered pilot is just screwin' around with it's centrifugally generated gravity..... so it won't hurl all over itself?

"The fact that the Pan STARRS-1 survey barely detected 'Oumuamua at its closest approach could be seen as an indication that there are many other such objects that were not detected, bolstering the case for 'Oumuamua being one of many such probes."

Doesn't that bolster the case for it being in a "random population of objects"? 10^15 is a huge figure, but I wonder how they are considering sampling bias if you will. I.e. that we only detected it because of its trajectory, and interstellar objects with less remarkable/close trajectories through our solar system are less likely to be detected.

Note that I think artificial origin is a serious possibility, and apparent tumbling could be for surveillance/communication (no need for a moving antenna when your planned tumble will eventually point you in any direction relatively soon during your 400,000 year journey), camouflage (to appear natural), artificial gravity, even an optical illusion or something else we don't understand yet.

@Spacebaby2001 I think they mean that the entire object is the sail, perhaps carrying a tiny payload like a CubeSat. Also they think that if artificial it could be broken and the spinning may not be intentional.

Agreed Spacebaby2001, they seem to completely ignore the measured rotation of the object. A rotating/tumbling space sail is not going to propel much of anything.

Disagreed, because, that is the design being used by the deep space probe sail which is intended--as the article pointed out. Given the current set of knowledge evident, and the metallic aspect, it is most likely one of our own.The sail would be 2 bags filled up from the central module, using centrifugal force of the spin--causing the appearance of tumble. And, if able sensors at the end would add a very good observation device for triangulation.This would explain why it was not observed on approach, and only after the bag-sails had engaged to carry it out of the solar system with an intergalactic travel path. ;) I'll probably get investigated, now, but I assure everyone I am not associated in any way with secret operations :) [Just one of the most intelligent people to ever exist, maybe, and you don't know me.]

Agreed Spacebaby2001, they seem to completely ignore the measured rotation of the object. A rotating/tumbling space sail is not going to propel much of anything.

Yeah it would propel... the net force is still forwards, torques while it's angled averaging out, the average propulsion would be half the maximum though (average of sin^2)...

Is in not possible that rotation of such a sail would be the best way to control it's speed and or steer it? what other braking or directional mechanism might it use, if not the surface area it directs to a solar body...

Oh my favorite part of all this kerfluffle are the speculations that the sail is invisible.

& the claims that the acceleration is being caused by outgassing of ices or dust, in quantities too small to be detectable. Trying to explain another claim, also not measurable, that the rock may have had an infinitesimal increase in acceleration, after passing the Sun. When Sol's gravity would be dragging it back.

Considering the possible range of guesses as to the asteroid's mass? Taking the low end of the scale? How much reaction mass would need to be expelled to increase propulsion?

Earlier, I had speculated if a Solar CME event had given Oumuamua a tiny shove? Enough to explain the rounding-error delta/v.

What I get out of this & previous articles about Oumuamua is first. That no one was actually observing this Cosmic Voyager before it passed our Sun. So there is a lot guesswork as to it's actual trajectory of entrance & speed into our System.

& the claims that the acceleration is being caused by outgassing of ices or dust, in quantities too small to be detectable. Trying to explain another claim, also not measurable, that the rock may have had an infinitesimal increase in acceleration, after passing the Sun. When Sol's gravity would be dragging it back.

There were speculations that the rock's path was bent, altering it's course while passing close to Sol. An alternative guess I thought of was. As Oumuamua dived towards our Sun? That approach added acceleration from the gravitational force of the looming Sun. Yet not enough pull as it whipped away, to prevent it's escape back into Interstellar Space.

no need of LGMs or BEMs to explain the sheer randomness of the eventful visitor.

Thanks jd, I did give it a quick skim. (yes, tongue-in-cheek) I need to go back and put more effort into understanding the authors assumptions.

However, I did immediately notice that the authors of the paper were careful to qualify that their conclusions are hypothetical. I don't see as they proved that my guesses are any less valid than theirs.

Unless I missed that part of their work where they included confirmed observations of Oumuamua as it entered the Solar System? Or, measured it's speed as it came close to the Sun? All those guesses are based on models of what they speculate occurred before perigee.

Wouldn't it be a real hoot if it turned out that all our speculations, the roller derby fling, outgassing, CME spanky-spanky, gravity assist by all the System's masses, dust-bunnies, all contributed to the claimed discrepancy?

jd, I did trudge my way through the paper you offered. And I simply do not find their expectations for a non-gravitational propulsion to be proven.

My suspicion is that a core group of the authors were gathered around a keg, brainstorming wild speculations and crazy ideas.

I do not understand why these researchers did not want to credit gravity assist but off the top of my head? Gravity is boring! If they could conceive an alternate mechanism? There would be a whole lot of professional status to be gained.

In their paper they made a fuss over how smooth the velocity measured for Oumuamua.

Yet they failed to identify any observed means of attaining this velocity.

Let's start with the EE/Eu woo. No one has observed any electrical discharges, or thundermugs for that matter, as Oumuamua passed through the Solar Wind.

Next, ridicule "Ludicrous Speed" is the invisible light sail. How does that even work, if it does not reflect photons?

Gee, what if it was highly electrically charged......and what if there was a strong magnetic field around the Sun?

Zilch. Let's see the maths. We know the field strength in the solar system. Invent a charge on the object (where are you getting it from? Why is it persisting?). Or are you just making crap up based on idiotic electric universe woo?

- cont'd -The authors of the proffered paper decided to call Oumuamua a comet instead of an asteroid. That is how they configured their models with selective data. The problem they have not resolved is the lack of any observable dust, coma or other form of exhaust.

However, I must credit their ingenuity in taking that idea, expounding upon it & doing a credible job creating viable scenarios for their hypothesis. Too bad there is no observed, physical evidence to support their conclusions.

The table displaying the estimated recent history for Oumuamua. From 1605 AD when it was at about 2300 AUs distant, traveling at about 26.34 km/s. By the year 2000AD it was within our Solar System and picking up speed. By 2016, it was 10 AU out at 29.5 km/s.

At 1 AU from our Sun, while crossing the Earth's orbit, it's speed had doubled. Did anybody see an exhaust plume?

- cont'd -What would the delta/v budget be, to speed up and slow down a megatonnage+ object? How enormous a visible cone of exhaust would be produced?

By perigee, the rock's speed was nearly doubled again. The steady rate of acceleration is simply explainable by the total gravitational pull of the Sun & planets.

Unless Oumuamua ran head on into the Sun or a planet? It's velocity was more than sufficient to pull out of it's dive in a hyperbolic orbit exiting our Solar System.

It was more than a month after shooting the Solar rapids that Oumuamua was spotted. As it again crossed the Earth's orbit, it's speed had already fallen by about half. Yet again no one has seen jets or other outgassing? Why not? Cause maybe, there wasn't any? Seems the most reasonable answer.

When you see the table as a whole, you will notice that the velocities approaching and receding match up dependent on the distance from Sol.

The total Gravity of the Sun & planets. In varying, constantly changing totals dependent on where the planets were in relation to Oumuamua's hyperbolic orbit skimming the Sun.

The total of Mass pulled the rock close to the Sun. Then the total of Mass began to deaccelerate Oumuamua as it sped out of the System. Taking back the extra acceleration temporarily loaned to the rock.

Insisting that there had to be an artificial mechanism begs the question where is there observed evidence for such a phenomena?

Even a small solid rock would use a huge amount of propellant.Are you sure you want to claim there was no Gravity involved?

The researchers listed in this article wanted to explore the possibility that this can all be explained by non-gravitational means. As elegant as their models? Those are not proof of anything but the modelers imaginations.

jd, honestly? If the article had been titled "Electromagnetic Plasma Propelled Oumuamua"? Would you feel so protective?

One error I made in my comments is, for"acceleration"? I should have used "momentum".

Looking back over the thesis, I wondered again why the author's rejected gravitational influence. I condclue that estimating the gravity tug by all the planets, major & minor. Against the attraction of the Sun would be very uncertain. Continuously changing hour by hour, as each planet moved in it's orbit.

In a continuous process of calculations of to what degree of relationship there is between each & every Mass. All these different gravitational sources contributing to the overall total to influence Oumuamua's course & momentum. Each adding fine tweaks to steer & pull.

As I was considering my opinion, a sneaky suspicion came to my fevered paranoid cynicism. That the authors had grown weary of all the nutjob fantasies they have had to endure? And came up with this paper as a satirical reply.

We've been punked!A revelation of the modern revival of the "cargo cult".

Read "Gravity Assist Engine for Propulsion" by Arne Bergstrom if you want a better theory of propulsion than a solar sail, particularly for an oblong spinning body. Basically, uses a three body tidal force to alter speed and direction, without chemical propellant or off-gassing.

Not good at reading comprehension are you? Might want to slowly go over what JD said and linked to:

"After ruling out solar-radiation pressure, drag- and friction-like forces, interaction with solar wind for a highly magnetized object, and geometric effects originating from 'Oumuamua potentially being composed of several spatially separated bodies or having a pronounced offset between its photocentre and centre of mass, we find comet-like outgassing to be a physically viable explanation"

Outgassing, not aliens or magically produced magnetic fields and charges.

Is a cometOr asteroidAs comet and asteroidIs ice covered dust with slabs of rockA comet is an asteroidIs a ComroidOumuamuaIs in angular spinWhere the solar wind Propels this colossal remnant ComroidAs a solar sail beyond planet PlutoSo as it spins like a topThe nonexistent pressureOf the solar wind Propels this ComroidComplete with spin crazed aliens To other planets to visitAs Christmas is a comingAnd the Comroids are getting fatSanta from his poleIs going for a alien spin To add his little additionTo add to this mystery That as Santa exists with his elf's and spirits in the poleMake these Comroid aliens The Christmas Conspiracy of 2018

It would seem to be more of a light pump using radiation pressure propulsion, than a light sail. Instead of outgassing, it may be gaining and losing "mass" in the form of absorbed and emitted light, perhaps flowing through hydrogen trapped within.

jd, I think that the authors of the "Non-Gravitational" thesis have created a null-hypothesis. Simply because of the lack of observed evidence to support their models.

No exhaust, no coma, not even souvenir photos of vacationing LGM tourists throwing rocks behind them.

In addition to the smooth velocity of momentum as Oumuamua approached, passed & receded from the Sun. With close but not perfect rate/scale of velocity at each AU from the Sun.

I do admire the authors ingenuity in creating their models. I am baffled at the cargo-cult insistence that they are describing reality.

Considering the constant process of changing gravitational influences by all the bodies of this Solar System? Of course there is going to be unpredictable interference upon the course & velocity of Oumuamua. As I said, these are rounding errors & need no exotic explanation than that the Universe is perverse.

If this is an interstellar asteroid or comet that has spent billions of years on a journey through space, finally wandering into our solar system... what are the odds of that? What is the probability that right now, in 2018, one of these things flies into the solar system?

It's not that the solar sail idea is scientific or unscientific. It's just an idea. If anything, the solar sail thing is a reminder how very important it is to solve the acceleration issue.

space, that's a very good question. The hard part would be creating the picket system that can detect such interstellar intruders before they get within a few tens of AU's of crossing through our system.

Oumuamua was not sighted until forty days after it's closest approach to our Sun and was already, rapidly receding back out across the Earth's orbit. "Up! Up! And, Away!"

Once we have a database accumulated of confirmed sightings? Then we could start a betting pool on what is next and when. Though I'd bet , not only would decisive sightings be a rarity. From any direction, at any possible moment?

Some will be loners, some will travel as packs. Some will be cometary icebergs and some will be bare rock and knapped flint as was Oumuamua. And some will be raucous carnivals of gravel and volatiles, that will disintegrate when Sol and Jupiter start playing tug-of-war.

But I do know that if Oumuamua is an asteroid that is changing its direction and speed like a comet, but without any visible indication such as off-gassing, then even NASA's completed "near earth object" safety survey isn't likely going to be very reliable or predictive.

I think this is why the unexpected behavior of the object is so important. I would like to see the odds of an interstellar object even coming into the solar system. I suppose it's like one every XXX million years.

The fact that it is so rare means that any unexpected behavior is super important.

Makes you wish that Oumuamua came like a thousand years after it did, when we might be better prepared.

Oumuamua was not observed approaching this System & not seen for another forty days after passing the Sun. Using the terms "unexpected" ", behavior", "rare" or "very important" is really stretching credulity out of all proportion.

The odds are that such visitors are an irregular event.It happens when it happens. Our corner of the Galaxy is full of debris. While the bouncing dice of random opportunity decide how often these remnants of stellar reproduction might pass close to us.

The problem is, we lack the technology to detect such events. We lack the orbital infrastructure that will be needed to establish a fleet of picket Early Warning Satellites to englobe this System as well as the Earth.

When you stub your toe on a rock? I bet you curse it for deliberately harming you!

Yes. yes, the Universe is out to get you, personally. Cause you are at the center of the Cosmos. Existence would be meaningless without you.

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